Woods Hole Oceanographic Institute has announced that its hybrid ROV, the Nereus, was lost over the weekend and is unrecoverable, having sunk to the bottom of the 10,000 meter Kermadec Trench.

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The 14 foot-long, 3-ton Nereus, which was capable of diving both with and without an optical tether, only recently joined the WHOI's robotic fleet in 2009 and at one time held the world record for deepest dive by an ROV—a distance of 10,902 meters in the Marianas Trench.

Our dive last night was the deepest one planned of the entire cruise—10,000 meters. Six miles. Everything was going fine. Two sets of pushcores were in the sample basket, and we'd recorded nearly two hours of video transects, plus more of individual animals that thrive in the hadal environment at the bottom of the Kermadec Trench. We'd just completed collecting a sea cucumber for the respirometer and were getting Nereus ready to head to the underwater elevator. Then the camera feeds abruptly went dark, and we lost communication with the vehicle.

We weren't particularly worried—it had happened before and the vehicle designers had planned for something like this. Whenever Nereus lost contact with the surface, it was programmed to wait 30 minutes so that the ship could be moved a safe distance away before Nereus dropped its ascent weights. So we moved and waited. But this time the acoustic communications modem on the vehicle, which is used to determine the vehicle's position from the ship, also went silent.

This still wasn't a troublesome problem. We knew its last position on the seafloor, roughly how fast it would rise, and the nature of the currents beneath the surface. So we knew approximately when and where it would appear. But it didn't. Groups of us stood as high on the ship's catwalks scanning the ocean as lines of squalls rolled through and blurred the horizon. Hints of concern began to appear as time passed, but again, it wasn't particularly troublesome because there were several more back-up release modes built into the vehicle. The last was a "dead-man" galvanic release holding the weights that was set to corrode away roughly 24 hours after Nereus went in the water.

We were nearing the time window associated with the first of those backup releases when one of the ship's crew spotted some white objects in the water. Then more. The rescue boat went in and three crew members began scouring the surface with nets as more and more white dots appeared. By then we knew. Nereus was gone.

That poor, poor sea cucumber. This is a blow to the WHOI's research efforts, certainly, but it could have been much worse. They could have lost the $42 million Alvin.